1. Field of the Invention
The present invention relates to a process for the production of a printed wiring board having plated through-holes.
2. Description of the Related Art
A pattern plating process and a tenting process are known as processes for the production of a printed circuit board having plated through-holes. The pattern plating process comprises subjecting the whole copper-clad laminate, in which through-holes have been formed by drilling, to an electroless copper plating process, to thus provide an electroless copper plating on the internal surface of the through-hole or the whole surface of the laminate, subjecting the whole laminate to an electrolytic copper plating until a copper deposit having a specified thickness is formed on the internal surface of the through-hole, by using the formed electroless copper deposit, covering the laminate with a resist pattern (a negative pattern) except for a portion to be formed as a conductor pattern, subjecting the laminate to pattern plating, removing the resist pattern, and conducting etching.
In this pattern plating process, electroplating is conducted only on a pattern, and therefore, as the pattern is usually uneven, the plating thickness is uneven over the plate, and this becomes an obstacle to the subsequent resist removing, etching, and formation of a solder resist.
On the other hand, the above-described tinting process comprises subjecting the whole laminate to electrolytic copper panel plating, patterning a resist pattern (a positive pattern) on a portion to be formed as a conductor pattern, in such a manner that the upper and lower openings thereof are covered, and then conducting etching.
In this process, when a pattern mask shifts during the exposure for the formation of a resist pattern, the deposit on the inner wall of the plated through-hole is etched, and thus an electrical continuity failure occurs. Therefore, a problem arises in that a high-density printed wiring board having a small plated through-hole land width cannot be produced at a high yield by this process.
Further, in this process, since the etching for forming a conductor pattern is conducted after the whole copper-clad laminate is subjected to an electrolytic copper panel plating, the etching depth is the whole height of the conductor pattern, and becomes inevitably great and thus the advance of the etching is significant during the etching, not only in the depthwise direction but also in the lateral direction under the resist. Accordingly, in the above process, the width of the formed pattern is inaccurate, and thus it becomes difficult to produce a fine pattern.
To solve the above-described problems, Japanese Unexamined Patent (Kokai) No. 61-2386 discloses a process which comprises a step of activating a copper-clad laminate having a through-hole formed therein and forming a pattern by etching while protecting an activated catalyst within the through-hole, a step of coating the laminate, except for a desired portion, with a predetermined solder resist, and a step of depositing copper through by using a predetermined electroless copper plating solution on the through-hole and the desired portion.
In this process, however, the conductor layer necessary for the internal surface of the through-hole is formed through an electroless plating alone, and thus it is necessary to conduct the electroless plating for a long time. There is, however, no known resist capable of forming a precise pattern and withstanding an electroless copper plating solution for a long time, and thus a high-density printed wiring board cannot be produced by the above-described process.
Further, in the above-described process, since the electroless copper plating is formed in a large thickness, the change in the composition of the electroless copper plating solution is significant during the electroless copper plating, and thus it is difficult to control the plating solution.
The present invention has been made to cope with the above circumstances, and thus an object of the present invention is to provide a process for the production of a printed wiring board, by which a pattern having a high density, a high fineness and a uniform thickness can be formed at a high yield.